Any unit whose parts-max is greater than the default of 1 is a
multi-part unit, and its hp denotes size rather than amount of damage.
Armies and fleets are two kinds of units which can be usefully defined
as multi-part.

Players will very often want to merge or detach parts of a multi-part
unit, and there is an action transfer-part provided for that.
You can control the cost of the action by setting
acp-to-transfer-part.

Side changing is like capturing, but players can only do it to units
that they control. The action is change-side, and you enable by
setting acp-to-change-side to 1 or more. This will also enable
side changing for units that cannot normally act.

Side changing is especially useful for alliances in multi-player games,
so it should usually be enabled. On the other hand, it should not be
too cheap; you should consider what side changing really means in the
game's context.

For instance, even in the close British/American alliance during WWII,
armies never actually changed sides; British ground units were always
British, and American ground units always American. On the other hand,
ships and bases could be traded back and forth with only a cost in time
and expense.

Sometimes a player will want to get rid of a unit,
perhaps because some type has been overproduced and is tying up
valuable resources, or to prevent it from falling into enemy hands.

You can allow this by setting acp-to-disband to 1 or more.

You can control the rate of disbanding with hp-per-disband.
You may, for instance, want to allow the deliberate destruction
of large units, such as battleships, but you don't necessarily want
disbanding to be a convenient way of preventing their capture.
Setting hp-to-disband so as to require several turns to
get rid of a unit will accomplish this.
The table supply-per-disband will allow you to govern the
rate of recovery of the unit's supplies during the disbanding process.

It is also possible to make disbanding a way to recover materials
that were consumed in the construction of the unit, by using the
table recycleable-material. Care should be taken that creation
and disbanding of units is not a convenient way to manufacture lots
of a material; players will use the loophole if it exists!

It should usually not be possible to disband something large like a city,
otherwise a clever player might try to eliminate it as a strategic target,
but most mobile units should be easily disbanded.
This is especially helpful in an "construction spiral" game, where
the winning player(s) can accumulate large numbers of useless units.